Aaboud, M.Aad, G.Abbott, B.Abbott, D.C.Abdinov, O.Abeloos, B.Abhayasinghe, D.K.Abidi, S.H.AbouZeid, O.S.Abraham, N.L.Albert, JustinAnelli, Christopher R.Chiu, Y. H.Ghasemi Bostanabad, M.Hamano, KenjiHill, Ewan ChinKeeler, RichardKowalewski, RobertLefebvre, Michelet al.2020-10-282020-10-2820192019Aaboud, M., Aad, G., Abbott, B., Abbot, D. C., Abdinbov, O., Abeloos, B., … Zwalinksi, L. (2019). Search for top-quark decays t → Hq with 36 fb−1 of pp collision data at s√s = 13 TeV with the ATLAS detector. Journal of High Energy Physics, 2019(5). https://doi.org/10.1007/JHEP05(2019)123https://doi.org/10.1007/JHEP05(2019)123http://hdl.handle.net/1828/12276A search for flavour-changing neutral current decays of a top quark into an up-type quark (q = u, c) and the Standard Model Higgs boson, t → Hq, is presented. The search is based on a dataset of pp collisions at s√ = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 36.1 fb−1. Two complementary analyses are performed to search for top-quark pair events in which one top quark decays into Wb and the other top quark decays into Hq, and target the H→bb¯¯ and H → τ+τ− decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ-leptons, is exploited in the two analyses respectively. Multivariate techniques are used to separate the signal from the background, which is dominated by top-quark pair production. No significant excess of events above the background expectation is found, and 95% CL upper limits on the t → Hq branching ratios are derived. The combination of these searches with ATLAS searches in diphoton and multilepton final states yields observed (expected) 95% CL upper limits on the t → Hc and t → Hu branching ratios of 1.1 × 10−3 (8.3 × 10−4) and 1.2 × 10−3 (8.3 × 10−4), respectively. The corresponding combined observed (expected) upper limits on the |λtcH| and |λtuH| couplings are 0.064 (0.055) and 0.066 (0.055), respectively.enHadron-Hadron scattering (experiments)Rare decayRare decayArticleDepartment of Physics and Astronomy